1. Field of the Invention
The present invention relates to using light sensors and computing devices to compute the depth of a scene. More particularly, the present invention relates to using light sensors and computing devices to compute the depth of a scene using shuttered light pulses.
2. Description of Background Art
The fields of optical range finding and optical range imaging are concerned with using light to determine the depth of a scene. Approaches that use light to determine the depth of a scene include interferometry, triangulation, phase shift measurement, and time-of-flight measurement. Interferometry techniques calculate the interference patterns caused by combining two beams of coherent, monochromatic light that have reflected off of an object. Triangulation techniques calculate a disparity map of a scene based on images of the scene generated from two viewpoints. Phase shift measurement techniques calculate the phase shift between a beam of intensity modulated light and that same beam after it has reflected off of an object.
Direct time-of-flight (TOF) techniques measure the time it takes a beam of light to travel from its source to the scene and back. Since light travels at a constant speed, once the TOF is known, the distance to an object can be determined. A TOF technique generally uses a short pulse of light. A light sensor is used to detect the arrival of the reflected light. TOF can be measured directly by using a clock to determine the amount of time that elapsed between when a beam was emitted and when its reflection arrived.
TOF can also be measured indirectly by analyzing the intensity of the reflected beam over time. This technique is known as “photon-counting.” Since light reflected from closer objects returns before light reflected from farther objects, knowing the intensity of the reflected beam over time enables scene distances to be determined. A light sensor can measure the amount of light that has hit it (known as “integrated light”), which is determined by the intensity of the light and its duration. One way to measure this quantity over time is by controlling when a light sensor can sense light (known as “shuttering” the light). Shuttering varies the integration time of the sensor and can yield a set of time-sampled values, which represent the intensity of the reflected beam over time. For example, if a light sensor stops sensing light before all of the incoming light has arrived, it will have captured more light from closer objects and less light from farther objects. As a result, nearby objects will appear to be bright, while farther objects will appear to be dim. This technique is known as shuttered light-pulse (SLP) ranging or SLP imaging and relies on the fact that the depth of an object is proportional to the amount of integrated light measured by a sensor.
Although the amount of integrated light measured by a sensor depends on the depth of an object, it also depends on several other factors, including the object's reflectivity, the amount of ambient illumination, and light scattering. An object's reflectivity, known more generally as “albedo,” represents the fraction of incidental light that is reflected by the object and varies based on the object's color and texture. Ambient illumination represents the amount of ambient light that is incident to the shuttered sensor. Light scattering is caused by shutter imperfections, which enable a shuttered sensor to sense light, even if the shutter is closed.
In general, in order to solve for each unknown value of a factor, a measurement from at least one independent shutter location is required. For example, in order to determine the depth of an object while also accounting for the object's albedo, measurements from at least two independent shutter locations are required. In addition, each sensor must be calibrated. Once the above-mentioned measurements have been obtained, they are used to determine the depth of the object. Existing SLP techniques do not adequately calibrate sensors and manipulate the above-mentioned measurements, especially when more than two shutter locations are used. As a result, the calculated scene depth is often incorrect.
What is needed is a shuttered light-pulse technique that adequately calibrates sensors and manipulates the above-mentioned measurements.